In the processing of semiconductor wafers, wafer heating is required to accomplish one or more device fabrication techniques such as rapid thermal annealing (RTA), temperature gradient zone melting (TGZM), lateral epitaxial growth on oxide (LEGO) or high temperature recrystallization (HTR). Such heating is typically accomplished by placing the wafer on the ends of a plurality of pins which project from the floor of an oven. A bank of tungsten lamps is mounted in the upper portion of the oven for heating one major surface of the wafer. The opposing major surface of the wafer is exposed to the floor of the oven. The floor of the oven is often cooled by cooling coils or the like so that the opposing major surface of the wafer is made cooler than the major surface of the wafer exposed to the heat from lamps. Thus, a temperature gradient is established through the wafer which is very desirable in achieving TGZM, LEGO and HTR. Examples of ovens of this type for heating semiconductor wafers are disclosed in U.S. Pat. Nos. 4,001,047; 4,041,278; 4,097,226 and 4,101,759.
Non-uniform lateral heating is generally undesirable since temperature variations across the wafer cause difficulties during wafer processing. For example, during heating of the wafer to accomplish temperature gradient zone melting, temperature variations across the wafer can lead to distortion of the migration pattern of the dopant. Uneven heating during the lateral expitaxial growth over oxide process can result in non-uniform melting across the wafer. More generally, lateral temperature variations across the wafer can result in substantial stresses on the wafer which can cause slip therein, that is, non-elastic deformation of the wafer lattice.
In an attempt to attain uniform lateral heating, known ovens may be provided with a diffuser situated between the wafer and the bank of lamps to uniformly distribute the heat across the wafer. Further, reflectors may be provided on the oven floor to reflect heat generally towards the wafer rather than to any specific portion of the wafer. However, with known ovens, uneven heating of the wafer often occurs because heat is lost from the edge of the wafer both by radiation and convection. It has been determined that more heat is lost from the periphery of the wafer by convection than by radiation except in a vacuum environment. Convective heat losses cause the periphery to be considerably cooler than the central portion as compared to the temperature difference between the periphery and the central portion of the wafer caused by radiation losses. The periphery of the wafer is defined as the generally annular area proximate the wafer edge.
Accordingly, there is a need for a technique for reducing lateral temperature variations across a semiconductor wafer during heating.